Iso rooms for trying out guitars/amps in are a must!! Nothing stupider than being serious about buying an instrument but having to compete noise-wise against some clown who you can tell absolutely isn't. I bought a guitar once from a small independent shop and some lady who had obviously never played a guitar before in her life decided it would be fun to play one in that shop, on that day, at top volume while I was trying to work out whether to buy a Telecaster... had the guitar not been a discontinued model that was hard to find at retail I would have walked out, it was so annoying!!!

Having a qualified service department is a must, too - you will probably see a lot more repeat business from me if you either do good work yourself or are able to send it out to someone reputable on a reasonably quick turnaround. Although I tinker with doing my own setups, once a year or so on average I'll pay for a pro job on all my guitars (that's 8 or 9 if you're counting, x £35-£50 for the average setup + strings... obviously I don't get them done all at once, but it could be a tidy earner for you if you do the job right.)

I would like to see shops offer "setup with purchase" too... I would love to be able to choose a guitar, then go for lunch/coffee while your tech puts some brand-new strings on it and adjusts everything to absolute perfection. I know a handful of mail-order/internet operations in this country do that before sending stuff out (helps minimise returns I suppose!) and it's something I really value highly. I do order guitars sight unseen sometimes, because I know I can send 'em back (yay distance selling regulations!) - but it's a service I would appreciate very much both online and in-store.

Hmmm... what else... the basics have already been pointed out - have a wide range of stock - have educated staff too - don't make sh*t up when I ask you questions. I might be testing you, I do a LOT of homework before even looking if I'm in the market for something. Be educated about all your brands too, and maybe I can learn something from you - maybe you can show me a brand that I wouldn't have otherwise considered if there's a genuinely interesting talking point about it.

I once bought a second-hand delay pedal from a shop who is not DV, and the coolest thing they did was let me try it out as long as I wanted in an iso room with any guitar/amp I wanted in the shop... I opted for a mid-range Les Paul at that time, but they were happy to let me play absolutely anything, and they were totally cool about it. That's important!